The present invention relates to systems for airborne terrain profiling, and more particularly, to laser rangefinding systems.
Airborne terrain profiling and surveying systems are used in a variety of topographic and mapping applications, such as stream-valley profiling. Such profiling and surveying systems typically include an aircraft-mounted rangefinder, or altimeter, system which operates in conjunction with a position-determining system to provide altitude readings corresponding to ground points underlying the aircraft flight path. The accuracy of the rangefinder system is critical to the precision of the resultant altitude readings.
In applications where there is sufficient time to average a large number of readings, the prior art rangefinder systems have been developed with continuous wave (CW) intensity modulated lasers. Intensity modulation is applied to the emitted light beam by way of an acoustic-optic or electro-optic modulator and range measurement is accomplished by electronically determining the phase shift of the return signal with respect to the transmitted signal, with high accuracy being established by utilizing a relatively high modulation frequency. For airborne terrain profiling systems, the CW approach is characterized by a substantial disadvantage in that there is a range ambiguity every 2.pi.radians of the modulation frequency. Generally, this ambiguity is resolved by using one or more low modulation frequencies, although, in a moving aircraft environment over random terrain which may contain sharp discontinuities in elevation, this technique is not fully effective. A further disadvantage of CW systems is the required heterodyning for transmitted and received waveforms to some lower intermediate frequency so that the phase measurement can be performed with a clock frequency which is relatively low. This necessitates relatively narrow bandwidth which may prevent the receiver from following and tracking rapid changes in phase resulting from sudden changes in terrain elevation. Furthermore, for CW systems, when the laser beam hits multiple targets such as may occur with foliage covered terrain, the phase of the return signal is a composite due to all intercepted targets, making the range measurement meaningless.
Pulsed laser systems are generally used for rangefinding where accuracies of only a few feet are required, such as in many military rangefinding applications, and in long range distance measurements, such as ranging to a retroreflector on the moon or on a satellite. In such systems, an unambiguous range determination is achieved by measuring the propagation time for a light pulse transmitted from a source to the target and reflected back to a receiver at the source location.
However, many pulsed laser systems are limited in application due to limitations in adjacent return pulse resolution. In low altitude airborne terrain profiling applications where the aircraft may be only 3,000 feet above the terrain to be mapped, for example, the conventional pulsed laser systems are particularly ineffective where the terrain may include foliage or other objects which tend to cause multiple reflections of a transmitted laser pulse. For example, when flying over trees, part of the beam may land on a branch or leaf and part of the beam might land on the ground. In such a case, two or more pulses might well be received in response to a single transmitted pulse, and only the last received pulse is representative of the range from the aircraft to the ground. Errors due to returns from multiple targets may be eliminated by detecting the last return pulse and using that pulse for terminating the time interval measurement.
Recently developed ultra-short pulse lasers and sophisticated signal processing techniques have partially overcome these problems and have permitted range accuracies on the order to several inches at such altitudes and under such conditions. Pulsed laser systems are however subject to substantial limitations due to dufficulty in processing the return signals because of variations in amplitude, rise time and shape, and the ever-present requirement to work with nanosecond and subnanosecond triggering circuits and time interval measurements in order to obtain high accuracy.
To compensate for variations in triggering level due to amplitude variations, a "half-max" detector, or constant fraction discriminator, has been developed. This discriminator provides an output pulse that occurs at a time when the amplitude of the received signal reaches a fixed percentage of the peak amplitude. This technique minimizes the variation in triggering point with amplitude changes as long as the rise time is constant.
In the prior art, last return pulse selection has been mechanized in several ways. One approach is to have each successive returned pulse strobe the contents of a time interval counter output on the fly and enter that value into an auxiliary register. However, in cases where high accuracy is required, for example, in an airborne survey profiling system, conventional time interval counters are not satisfactory, since sub-nanosecond interval counters utilize analog interpolation, and as a result, finite time is required to perform the complete time interval computation and provide the data to a register.
An alternative approach in the prior art has been to employ a pair of time interval counters, the first counter measuring the time from the start, or transmitted, pulse to an arbitrarily generated measurement cycle termination pulse that occurs at a time that exceeds the last possible expected return pulse, and the second counter measuring the time from the stop, or last, pulse to the same cycle termination pulse. The difference in the two counter measurements is the desired time interval from the start to the stop pulse. In the case of multiple returns, each successive stop pulse instantaneously resets and starts the time interval measurement so that the second counter measures the time from the last pulse to arrive at the termination pulse. However, such systems are subject to substantial cost limitations due to the requirement for two precision time interval counters. Furthermore, such counters are relatively large in size, weight and power consumption and consequently, do not lend themselves to airborne applications.
It is an object of the present invention to provide a high accuracy and high resolution rangefinding system suitable for use in airborne terrain profiling.
Another object is to provide a high accuracy and high resolution laser rangefinding system characterized by minimization of multiple reflection noise.